fix(utils): reject trailing characters in numFromString

[slideglide]

this PR is dedicated to ery

• fixes a bug where numFromString returned an Ok for strings containing valid numeric prefixes followed by unparsed trailing characters
• returns the message of the error code instead of returning "Unknown error"
• trailing data error returns "String contains trailing data" instead of "String contains trailing extra data" (extra is unnecessary here imo)

see the screenshots for reference